1. Field of the Invention
The invention relates to a metallically gross layer decorative molded article for protecting a radar device, particularly to a metallically gross layer decorative molded article placed in the beam path of a radar device installed behind the front grill of an automobile.
2. Background Art
Generally, the antennae for the transmission or reception of radio wave on communications equipment or radars, for example, are not much limited in terms of their design regarding the antenna body or its surrounding structures because of the priority placed on their functionality. For example, in the case of automobile antennae for the onboard radio, the bare rod antenna is used. However, it is sometimes desirable to hide the antenna, depending on its location. For example, in the case of a radar and the like for monitoring obstacles in front of the automobile or measuring its distance from the car in front, it is desirable to install the antenna at the center of the front of the car for optimum performance. In such cases, the antenna would have to be installed in the general area around the front grill of the car. In view of design considerations, however, the antenna should be invisible from the outside even in such cases.
In a technology called “auto cruise system,” a sensor mounted at the front of the vehicle is used to measure the distance between one's own car and the car in front or the relative speed. Based on the thus acquired information, the throttles or brakes are controlled so as to accelerate or decelerate the vehicle and control the vehicle-to-vehicle distance. The auto cruise system is gaining attention as a corner stone of the so-called intelligent transport system (ITS) that aims to reduce traffic jams and accidents. The sensors used in the auto cruise system are generally provided by radio wave transmission/reception devices, such as millimeter wave radars.
FIG. 4 shows a radar device 100 mounted on an automobile, which device is usually installed behind a front grill 101. The front grill 101 mounts an emblem 102 of the manufacturer of the vehicle or other unique ornaments. The millimeter wave is emitted by the radar device toward the front via the front grill and the emblem, and the wave reflected by an object returns to the radar through the front grill and the emblem.
Therefore, the front grill, emblem, and other portions in the beam path of the radar device preferably employ materials and paints that have little wave transmission loss and provide a desired appearance.
The radio wave transmission/reception device is thus generally mounted behind the front grill of a vehicle. The front grill, however, is often plated with metal, which is a good electric conductor and does not allow for easy transmission of radio waves. The front grill is also provided with ventilation openings for the intake of air, resulting in uneven thicknesses. If radio waves are transmitted through such a front grill, differences are caused in the transmission rate of the wave between the thinner and thicker portions of the front grill, thereby making it difficult to obtain good radar sensitivity.
Under such circumstances, it is common practice to provide a window through which radio wave can pass at a location of the front grill that corresponds to the location of the radio wave transmission/reception device. By providing such a window in the front grill, radio wave can be transmitted and received through the window. However, the window breaks the sense of continuity in the exterior of the front grill. It could also allow the interior of the vehicle, such as the radio wave transmission/reception device or the engine room, to be viewed, thereby damaging the appearance of the vehicle.
In a conventional technology, to solve this problem, a radio wave transmitting cover as disclosed in JP Patent Publication (Kokai) No. 2000-159039 A is inserted in the window portion of the front grill so that a sense of unity can be established between the window and the front grill. The radio wave transmitting cover disclosed in JP Patent Publication (Kokai) No. 2000-159039 A is formed of a laminate of a plurality of resin layers formed with irregularities. Such a covering component, which has a metal layer with irregularities vapor-deposited between the resin layers, can provide the impression that the fins of the front grill are continuously existing in the radio wave transmitting cover.
As a metal that is vapor-deposited on the radio wave transmitting cover, indium is used. When indium is vapor-deposited on a deposited member, indium is deposited on the surface of the deposited member not in the form of a uniform film but of fine islands. Specifically, when indium is deposited on the deposited member, the surface of the deposited member is rendered into a state where a fine island-like indium deposited portion and a non-deposited portion finely exist together. In this case, radio wave can be transmitted through the non-deposited portion, and yet because of the fine island-like indium deposited portion, the surface of the deposited member is viewed as a member having a metallic luster.
However, indium is an expensive metal material and using it for deposition results in an increase in material cost. Furthermore, it is difficult to form the deposited portion and the non-deposited portion with desired proportions. For example, when the deposited portions are formed very close to one another, the entry and exit of radio wave could be hindered.
JP Patent Publication (Kokai) No. 2003-252137 A discloses a method for manufacturing a radio wave transmitting cover that is disposed on the front grill of a vehicle that has a lattice-like fin portion, wherein a radio wave transmission/reception device is installed behind the front grill. In this method, a metal covering film can be formed using various metal materials other than indium and excellent exit and entry of radio wave is ensured. The method comprises: a first step of forming a planar cover substrate; a second step of forming a metal thin film on one surface of the cover substrate, the metal thin film having a shape corresponding to that of the fins; a third step of forming an island-like metallically gross portion by physically or chemically removing a part of the metal thin film; and a fourth step of forming a transparent resin layer on the upper layer of the surface in which the metallically gross portion has been formed in the cover substrate.
JP Patent Publication (Kokai) Nos. 2000-159039 A and 2000-49522 A disclose that a plastic covered component located within the beam path of the radar device is deposited with a thin metal layer consisting of indium in an area of a metal-made portion that is visible from the outside. In this case, however, it is necessary to ensure the durability and reliability of the metallically gross design and the radio wave transmitting property by forming a stable protection layer on the indium metallically gross film layer, so that the indium metallically.
This is due to the following reasons, among others. For one thing, indium is a very soft metal material, with a Mohs hardness of 1.2. For another, because indium is basically a metal material, it corrodes in the aforementioned environment. Also because indium is basically a metal material, it produces a radio wave transmission loss in the form of conduction loss, which makes it necessary to ensure durability and reliability by reliably ensuring a film thickness such that the indium metallically gross-tone design can be obtained without increasing the thickness too much. And yet for another thing, the melting point of indium is very low at 156° C., such that the indium layer is melted by the heat of molten resin during the secondary formation of a lining resin following the formation of a resin molded article on the surface of the substrate in advance.
In this connection, JP Patent Publication (Kokai) No. 2000-344032 A discloses that a ceramic film comprised of a silicon dioxide is provided as a protection layer for protecting the indium film from peeling and ensuring the corrosion resistance thereof.
It is also noted that, in order to prevent the erroneous reception of radio wave by the radar device, the exterior component of the radar device that has a metallic luster needs to attenuate the radio intensity as little as possible. Namely, in order to prevent the attenuation of radio wave by the exterior component itself, the refraction and reflection of radio wave between the materials and by metal films need to be minimized.